Automatic repeat request (ARQ) is a transmission scheme that a receiver sends feedback to a transmitter informing that a data block has been decoded successfully or not, and a failed data block is retransmitted by the transmitter based on the feedback. Hybrid ARQ (HARQ) is a variance of the ARQ. In HARQ, the previously failed data block is stored in the receiver and combined with a retransmitted data block.
There are two types of HARQ schemes: chase combining and incremental redundancy. In chase combining, when the receiver detects an error in the received data block, a retransmission is requested and the incorrectly decoded data block is stored and combined with the retransmitted data block. In incremental redundancy, the retransmitted data block is coded differently from the previous failed data block, rather than simply repeating transmission of the same data block as in chase combining. Incremental redundancy gives better performance since coding and modulation are effectively performed across retransmissions. Chase combining may be considered a type of incremental redundancy.
HARQ may be used in a stop-and-wait mode or in a selective repeat mode. In the stop-and-wait mode, one data block is transmitted at a time. After each data block is transmitted, the transmitter waits until feedback, (i.e., positive acknowledgement (ACK) or negative acknowledgement (NACK)), is received. A new data block is transmitted, (or the previous data block is retransmitted), only after the feedback is received or if a timer expires. In the selective repeat mode, the HARQ process continues to send a number of data blocks specified by a window size, regardless of the feedback (ACK or NACK). The receiver keeps track of the sequence numbers of the data blocks received and informs the transmitter in the feedback. Once the transmitter has sent all the data blocks in the window, the transmitter re-sends failed data blocks indicated via a feedback channel.
In a simple stop-and-wait mode, the transmitter has to wait for the receiver's acknowledgement and this reduces system efficiency. Therefore, multiple stop-and-wait HARQ processes are usually used in parallel, wherein one HARQ process is waiting for an acknowledgement, another HARQ process may use the channel to send data.
A universal mobile telecommunication system (UMTS), (such as high speed downlink packet access (HSDPA) and high speed uplink packet access (HSUPA)), uses HARQ with chase combining or incremental redundancy with multiple stop- and wait HARQ processes in parallel. For example, for HSUPA enhanced dedicated channel (E-DCH) transmissions, a medium access control (MAC) layer in a user equipment (UE) performs E-DCH transport format combination (E-TFC) selection and delivers a transport block to the HARQ process, which handles transmission and retransmission of the transport block. For HSUPA, for the E-TFC selection, the MAC layer takes into consideration the maximum allowed puncturing, the maximum allowed UE transmit power, and the serving grant for the E-DCH, (i.e., how much power is allowed to be used by the E-DCH). However, for retransmission of a failed transport block, the same E-TFC is used, which implies that the same transport block size is used.
This causes several problems. First, the level of puncturing may be different for retransmissions in case the compressed mode used in the given frame is different. This may lead to higher puncturing, which may cause the UE to exceed its allowed puncturing, (i.e., the puncturing limit). Second, the power used by the E-DCH dedicated physical data channel (E-DPDCH) depends not only on the block size but also on the compressed mode used in the frame. The transmit power is recalculated for each retransmission, (i.e., beta factors for E-DCH are adjusted for every retransmission based on compressed mode). If the transmit power required for the retransmission is higher than the initial transmission power, the UE may exceed its maximum allowed transmit power, in which case the power is clipped to the maximum allowed power. This will result in an increase of the probability of error in the data block and consequently in an increase of the probability that the transmission will fail. The UE may also exceed its E-DCH serving grant. This will result in an increase of interference in the cell, which may affect the overall system capacity.
In a system where adaptive modulation and coding (AMC) is used, (such as 3GPP long term evolution (LTE) system), for a particular allocation of radio resources, a less robust modulation and coding scheme (MCS) allows for larger transport block sizes and a more robust MCS requires smaller transport block sizes. As a result, since the transport block size is fixed for every retransmission, the transmitter may not be able to change the MCS between retransmissions.
For LTE, it has been proposed to re-segment radio link control (RLC) protocol data units (PDUs) or RLC service data units (SDUs) if a transport block containing the RLC PDU or SDU is not transmitted successfully. However, it is proposed to be done after the HARQ process has already tried to transmit the transport block, (i.e., after all HARQ level retransmissions allowed in the specific HARQ process take place). The PDU or SDU re-segmentation is not performed at an HARQ level, but at an RLC level, which means every HARQ level retransmission is performed using the same transport block size. Therefore, the LTE system would suffer from similar problems described above.